Thirty-eight patients may have been put at risk of Creutzfeldt-Jakob Disease (CJD) during surgery in south Wales.
Health chiefs confirmed it was two years after the operations between 2007 - 2009 that patients were informed.
Abertawe Bro Morgannwg health officials realised a patient who had surgery in 2007 was at high risk of developing the incurable brain disease.
Doctors say it is possible proteins which cause CJD were not destroyed during instrument sterilisation.
Thirty-seven of the patients are from mid- and south-west Wales and one is from north Wales.
CJD is a rare fatal disease that causes degeneration of nervous tissues, for example in the brain and spinal cord.
The initial operation took place in one of four hospitals - Singleton or Morriston in Swansea, Neath Port Talbot General or the Princess of Wales in Bridgend.
Health officials are not naming the hospital to protect the identity of the high-risk patient.
That patient first underwent surgery in 2007. When the individual returned for a subsequent operation in 2009, it was identified that the patient was at high risk of contracting the disease.
Officials then realised that between 2007 and 2009 surgical instruments that had been used in the initial procedure were used in 38 subsequent operations.
Letters were sent on Saturday to those former patients who, officials say, are at "extremely low risk".
Public Health Wales said there had only ever been six cases worldwide where any form of CJD had been transmitted in this manner.
Dr Jorg Hoffmann, PHW consultant in communicable disease control, said: "In this incident we do not have a single confirmed case of CJD.
"The [high-risk] patient does not have CJD and remains well without the symptoms.
"However, it is possible that the proteins that cause CJD, known as prions, survived routine sterilisation procedures so an extremely small risk of transmission remains.
"We have identified and written to all patients concerned to make them aware of the extremely low risk."
He said the patients have been offered information, a helpline and access to specialists in the field.
'Worried and upset'
Dr Hoffman said he had already spoken to a number of the patients and they were obviously "worried and upset".
"We do understand that it's very upsetting for the patients to live with this uncertainty for years to come," he added.
"Our advice is they should carry on to live their lives as before."
He said anyone who had any type of surgery in the ABM health board area since 2007 but had not been contacted had no reason at all to worry.
Once it was identified in 2009 that the initial patient was at high risk of CJD, PHW contacted the UK CJD incidents panel made up of experts from around the UK.
In February 2011 the panel said the health board should contact the 38 patients involved.
Dr Hoffman said there was "no suggestion of any wrongdoing by anyone".